Photosensitive elements which can be used in image-reproduction processes are well-known in the graphics arts industry. Such elements are exposed to actinic radiation through an image-bearing transparency, such as a color separation transparency, to produce an image which is either positive or negative with respect to the transparency used. Positive-working elements produce a duplicate image with respect to the transparency through which they are exposed. Negative-working elements produce an image that is the reverse of the transparency through which they are exposed.
After imagewise exposure, the photosensitive elements may be developed by washout of soluble image areas, toning of tacky image areas with a colorant, peeling apart of photoadherent layers, or combinations of these techniques. The resulting image-bearing photosensitive elements are particularly useful for color proofing applications. A useful reference for color proofing methods is Principles of Color Proofing, by Michael H. Bruno, GAMA Communications, Salem, NH, 1986.
In Cohen and Fan, U.S. Pat. No. 4,282,308 there is described a negative-working photosensitive element which is capable of producing colored images by a dry process without the need for toning with a colorant. The element comprises, in order from top to bottom, a strippable coversheet, a photoadherent layer containing a colorant, a tacky essentially nonphotosensitive elastomeric layer, and a support. After imagewise exposure to actinic radiation through the coversheet, the element can be developed by peeling off the coversheet. The exposed image areas of the colored photoadherent layer adhere to and are removed with the cover sheet. The unexposed areas of the colored photoadherent layer remain on the supported elastomeric layer.
When the imagewise exposure is through a negative transparency, and the exposed element is peeled apart, a positive image is obtained on the peeled-off coversheet and a negative image is obtained on the supported elastomeric layer. Imagewise exposure through a positive transparency, followed by peeling apart of the element, yields a negative image on the peeled-off coversheet and a positive image on the supported elastomeric layer.
Taylor, U.S. Pat. No. 4,489,154, describes a dry process for preparing a surprint proof without the need for toning with a colorant. This process uses an imagewise exposed, peel-apart photosensitive element comprising a strippable coversheet; a photoadherent layer containing a colorant; an essentially nonphotosensitive elastomeric layer; and a support. After exposure to actinic radiation through a positive image, e.g., a positive separation transparency, and peeling off the coversheet, a positive colored photoadherent image remains on the supported elastomeric layer. This image is then adhered to a substrate. By repeating this process with different colored photosensitive elements, a multicolored surprint proof can be built up on the substrate. If exposure is through a negative image, e.g., a negative separation transparency, a positive image is obtained on the coversheet.
It is sometimes desired to have a process which produces an image which is the reverse of that described above, i.e., after exposure the areas which are not imagewise exposed adhere to the coversheet rather than to the elastomeric layer so that these areas are removed with the coversheet, and the imagewise exposed areas remain on the elastomeric layer. If the element were precolored, imagewise exposure through a positive separation transparency would produce a positive overlay proof directly on the coversheet. As defined herein, a positive overlay proof is an overlay proof produced from a positive separation transparency. The precolored element would also be particularly useful for the preparation of negative-working, multicolor surprint proofs without the use of toners and which do not require the additional complication of an image transfer step and the addition of an adhesive layer.
While it may be possible to change the adhesive relationship of the unexposed and exposed photohardenable layer with respect to the other layers of the element, this is not an easy task. The multilayer peel-apart photosensitive element requires a very exacting balance of adhesive/cohesive forces in order to function properly. The addition or the substitution of new chemicals for ones previously used may indeed change the relative adhesive/cohesive properties but may, at the same time, adversely affect other important properties of the system. It is, therefore, desirable to achieve image reversal by alteration of the imaging process.